M20 firing order reversal

[Simon13]

Hi all, here is my condundrum on my M20 engine build with my shit schrick camshaft.

If you could imagine the only way to time up said M20 is by having the cam 180 degrees out to TDC then would the firing order need to be reversed to get it to run?

As the engine won't fire with the HT leads set in the stock format from last time it was built so i'd need to switch the HT leads about on the dizzy to get her to run i think.

Stock firing order is 1-5 3-6 2-4 according to bentley so it would need to be 5-1 3-6 4-2 run with the cam 180 degrees out no?

i can't get my head quite around it at the moment!

[DanThe]

Set the cam properly with a vernier would be the simplest way IMO

[bookmark]

Replace the cam?

[Simon13]

i've two cam sprocket holes on my cam. If i use the "proper" one the cam timing is 180 out. If i set it to the other hole like last time the inlet valves hit the pistons. Not sure your vernier will have enough retard in it to get a safe distance so that there is no contact and the cam is dialled in correctly.

If i go the other way for the sake of re arranging the HT leads then i'd rather this

[Simon13]

thanks will,

i'm getting a 3rd party to set the cam up with vernier and ive the specs from schrick, it will run this time

[Simon13]

Correct thats my plan, with the cam set to being 180 out as such its set to the same as TDC but just 180 out. The machine shop are going to dial the cam in the with vernier and the shcrick info i have and i'll adjust the HT leads on the dizzy to the order you have said and she should go.

[DanThe]

Cant you just unbolt the centre of the vernier and refit it 180° round so the timing mark is in the usual position? Would make it far simpler IMO

[HairyScreech]

So the cam is 180 out and the crank is effectively 360 out right?

In which case as Dan just said, recutting the TDC mark may be the best way around this.

Am I right in thinking this is the fabled a u t o m a c cam where the nose was drilled wrong?
If so it may be better to have the pulley markings altered on a standard fixed pulley after determining the correct position of the cam. While you would have 2 timing marks on the pulley one could easily be rendered illegible to prevent a timing mistake.

Changing the firing order will certainly do no harm, I have seen this done on yank V8s to for example run a chevy ignition system on a ford v8 with a modified cam. You should be able to do this to confirm your suspicions.

[Simon13]

this is that famous cam Simon! The two holes are not exactly opposite each other on the end of the cam



Top hole is the one shrick say to use, this times the cam up 180 degrees out and no valve contact

bottom hole which had the plug in it gets the cam timing correct but the inlets slap the pistons.



Would switching the vernier around cure the ignition though? As at TDC its pointing the opposite way to normal? But the leads are in the correct order so it will run as normal?







So switched 180





With a stock pulley





With the vernier, so the cam is correct both lobes shut on No.1, the ignition leads in the normal order, just the rotor is opposite to how it would sit normally - That won't matter at all and it will start run as normal right?




With stock pulley on, valves are open on no.1 so in order to run the firing order would be altered on HT leads.


The top option is better no? I can give it to them like that and they can dial the cam in and i'll be in business again 3 years on or whatever it is. And more simple to remember in the long term. Feels like we've cracked it!

[DanThe]

I forgot about the rotor arm!

It will need to be in the stock position relative to the cam lobes which will be a problem if the locating hole is out on the cam, so you were probably right first time

[Simon13]

this is an old donkey engine!

[HairyScreech]

What ever you do your timing is going to be 180 out as the rotor is going to be out of phase to the cam (by 180).

A combination of 180-ing the vernier to get the timing mark in the right spot for cylinder 1 and swapping the HT leads (1-6, 2-5, 3-4) to suit the new distributor position should see you right.

Your rotor in option 1 would be pointing to number 6 (had to work this out for the M3.3.1) instead of 1 which would be on the firing stroke. Put lead 1 onto cap position 6 and off you go.

Then make a note of the lead order along the top of the radiator

[HairyScreech]

On a belt engine I would always advise having a usable TDC mark for the cam.
In three years time when your mid way through a belt change and unsure if you just nudged the cam or not your going to be REALLY thankful for that TDC mark.

I learned this the hard way using a 1600 inlet cam on a turbo zetec.

[HairyScreech]

It also prevents the odd mishap where you bump the cam when removing the belt and dont notice it until you check the timing after fitting the belt, then have to redo the whole sorry mess... don't ask..

Been there, done that, taken 6 hours to change a belt in miserable drizzle on the drive.

[DanThe]

Timing marks are massively important, means you havnt got to rely on your memory